ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 953 



twenty-four hours, and coloui'ed with carmine for about twenty minutes, 

 the picric acid acting as a mordant. 



Transferring Sections from Alcohol to another fluid. * — In 

 order to avoid shrinking in transferring from absolute alcohol into 

 ethereal oil, or chloroform f (from a lighter into a denser fluid), Dr. W. 

 Giesbrecht recommends that a quantity of absolute alcohol should be 

 poured into a glass vessel, and by means of a pipette let the oil or 

 chloroform rim underneath it, so that the two fluids may lie one under 

 the other ; then drop the object into the alcohol, and take away 

 all the superfluous alcohol. When the objects have sunk to the 

 bottom of the vessel, the exchange of fluids is completed. Many 

 objects, however, will not sink down in the dense chloroform ; but 

 this inconvenience can be got rid of by a suitable addition to the 

 chloroform (e. g. sulphuric ether, &c.). In other cases the disappear- 

 ance of those figures of refracted light, which always make their 

 appearance when two fluids of different refractive powers mix, must 

 serve to indicate the completion of the exchange. This is the most 

 simple way to retard the exchange of fluids, and so avoid the cause of 

 shrinking. 



Imbedding in Paraffin and Freeing the Sections.! — Dr. Gies- 

 brecht also describes the process which he has found advantageous for 

 imbedding the object after its transfer, as above described, from 

 absolute alcohol into chloroform, which of all solvents of paraffin § 

 gives the best results, and is most easily evaporated. Some sul- 

 phuric ether should be added to prevent the object swimming, and 

 the chloroform, with the object, is then slowly warmed to the tem- 

 perature of the melting-point of paraffin ; whilst this is being done 

 small pieces of paraffin are put in gi-adually. Here, too, shrinking 

 is avoided, by making the displacement of the chloroform by the 

 paraffin take place very gradually. It is completed when no more 

 air-bubbles rise from the object. 



For fixing the section with certainty and facility during the 

 removal of the paraffin, a stock of slides should be provided, the 

 centres of which are overlaid with a very thin and very uniforra layer 

 of shellac. Such a layer is easily produced by dipping a pretty thick 

 glass rod into a well-filtered and not too concentrated solution of brown 

 shellac in absolute alcohol, and passing it lengthways over a slide 

 previously ivarmed. Shellac as clear as possible will of course be 

 chosen ; the so-called white shellac cannot, unfortunately, be used, 

 because it is not soluble in alcohol. Before commencing cutting, 

 brush over the shellac layer very tfiinJy with creosote, |1 and then lay 

 the section upon it with as little paraffin as possible. The slide, with the 

 section, is afterwards exposed for about _a quarter of an hour in a water 



* Zool. Anzeig., iv. (1881) pp. 483-4. 



t This applies also to transfers from water or alcohol into glycerine. 



j Loc. cit. 



§ A solution of very hard paraffin in an equal volume of chloroform keeps 

 fluid with the warmth of the hand. 



II Creosote dissolves both shellac and pamffin, hence its application in this 

 case. Turpentine does not dissolve shellac. 



